In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh wire or fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The papermaking machine is essentially a de-watering, i.e., water removal, system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
A wide range of chemicals is utilized in the papermaking stock furnish to impart or enhance specific sheet properties or to serve other necessary purposes. Such additives as alum, sizing agents, mineral fillers, starches and dyes are commonly used. Chemicals for control purposes such as drainage aids, defoamers, retention aids, pitch dispersants, slimicides, and corrosion inhibitors are added as required. Fabrication of quality paper requires addition of the proper amounts of these chemicals.
Many of the additives are removed in de-watering process; however, others such as ash remain in the final paper product. Generally speaking, ash is defined as the residue remaining after complete combustion of paper. Ash can include various materials. Many paper manufacturers use clay, titanium dioxide (TiO2) or calcium carbonate (CaCO3); and in some cases barium sulfate and talc also comprise ash. In some cases only one of these materials will be used, whereas some manufacturers use mixtures of these materials, a common combination being clay and titanium dioxide or clay and calcium carbonate. During the manufacture of paper, it is important to control the ash content of the paper. The concentration of ash can affect the strength of the paper and also certain qualities such as printability. Furthermore, clay, which is often a component of ash, is generally far cheaper than wood fiber. Therefore, it is often important to maintain the ash content as high as reasonably possible while still maintaining other characteristics of the paper within specification.
On a related aspect of papermaking, it is often desirable to coat a paper sheet (called a “base sheet”) with any of a wide variety of materials. Indeed, an increasing proportion of the world's paper production is devoted to coated paper and coated paperboard. Coatings are usually applied to provide a glossy white surface for magazine pages, gift wrapping, shoe boxes, and the like. Alternatively, or in addition, such coatings may also be intended to render the paper sheet waterproof.
There are a large variety of coating formulations, many of which consist of as many as ten or more components. These components can be broadly classified as pigments, binders, and additives, almost always as aqueous dispersions. Common pigments include clay, calcium carbonate, barium sulfate, and titanium dioxide. Barium sulfate and titanium dioxide are used primarily for photographic papers and specialty papers, respectively. Generally speaking, clay has been the most common pigment, although CaCO3 and PCC (precipitated calcium carbonate) are becoming more common. Various formulations of latexes are commonly used for binders to hold the pigment particles together and to bond them to the paper. A typical coating formulation includes 80% to 90% pigment, 3% to 10% latex, with the remainder consisting of additives or other components.
It is often desirable to obtain measurements of selected components of sheet materials during manufacture. Various sensor systems have been developed for detecting sheet properties “on-line,” i.e., on a sheet-making machine while it is operating. Typically, on-line sensor devices are operated to periodically traverse, or “scan,” traveling webs of sheet material during manufacture. Scanning usually is done in the cross direction, i.e., in the direction perpendicular to the direction of sheet travel. Unfortunately, obtaining an accurate cross-direction, i.e., cross-web, or machine-direction profile of a minor individual component or additive, such as CaCO3 or latex which are used as ash or coating component, is typically not feasible with conventional techniques such as x-ray, infrared absorption or x-ray fluorescence. For example, fluorescing dyes are of limited use because of low signal to noise ratios, low photostability, and lack of sensitivity. In particular, many fluorescing dyes have broad emission spectra and narrow absorption spectra thereby limiting the number of components that can be detected. In addition, it is difficult to discriminate the fluorescence associated with a particular dye given the high background and the broad emission spectra of the various dyes. Further, in the case of CaCO3, when the component is present in both the base sheet and the coating, two separate sensors are needed to perform the measurements. Finally, specialty paper makers often include components that do not exhibit unique signatures and therefore are not readily detectable by conventional techniques.